Current research is identifying an increasing number of membrane transport proteins involved in important physiological events, yet many questions remain concerning their structure and mechanism of function. UhpT, the phosphate: sugar-6-phosphate antiporter for Escherichia coli, is a prototype for secondary-type carrier proteins which is well suited to providing answers to these questions. In this study, the topology and the solute transport pathway of UhpT will be defined at its C-terminus using various sulfhydryl-reactive probes to analyze a single-cysteine library spanning transmembrane segments (TM) 11-12. Through the use of disulfide cross-linking and cysteine-targeted spectroscopic probes, this library will also be used to determine the relative positions of these two segments with respect to each other and to the well characterized TM7. Data obtained from this study will provide a starting point from which the structure of the hydrophobic core of UhpT can be inferred. Since UhpT shares strong similarities with secondary-type carrier proteins from both prokaryotes and eukaryotes, a model of its core structure and transport pathway should prove useful in developing an understanding of how these other systems are assembled and how the might perform. This is especially significant in light of the fact that many of these proteins play important roles in metabolism, homeostasis, and antibiotic resistance.